Environmental considerations require purification of industrial offgases containing atmospheric pollutants such as sulfur oxide emissions. Customarily, sulfur oxides are removed or scrubbed from industrial offgases by using lime, slaked lime or other calcia-containing compounds which are capable of removing the sulfur oxides from the offgas by reacting with the sulfur oxide pollutants. The calcia-containing scrubbing medium, through contact with the sulfur oxides, is converted to a calcium sulfate product, which due to its lack of further sulfur oxide conversion capability, is removed from the scrubbing system.
The fact that calcia-containing compounds are suitable for the removal of sulfur oxide pollutants from industrial offgases generated an interest in other calcium-containing compounds which could replace the presently employed materials. These substitutes have to meet the criteria of being readily available, capable of efficiently removing sulfur oxides from offgases and last but not least to provide an economic advantage over the presently available manufactured scrubbing medium. Cement plant waste product, containing some calcium oxide besides other calcium compounds became a logical replacement candidate for sulfur oxide scrubbing.
Cement plant waste product (CWP) contains calcium silicates, calcium aluminates, calcium carbonate and calcium sulfate, apart from calcium oxide and it results from the calcination of cement precursors. It is generally recovered in essentially anhydrous fine powder form from cement kiln dust recovery systems and not at least due to its fineness, it is usually considered as a waste product rather than a finished cement product. This CWP is available in large quantities, its utilization, for example as sulfur oxide scrubbing medium, would eliminate a serious waste disposal problem and would also provide an economic alternative to lime or slaked lime. Unfortunately, CWP was found to lack satisfactory sulfur oxide scrubbing or conversion ability and thus could not be employed for efficient scrubbing of offgases.
Surprisingly, it has now been discovered the CWP can be utilized as an efficient sulfur oxide scrubbing medium, provided it is, prior to its use in a sulfur oxide scrubbing system, preconditioned with liquid water. This preconditioning involves the essentially total rehydration of the rehydratable components of CWP, followed by drying at a moderate temperature and subsequent employment of the preconditioned CWP for removal of sulfur oxides from industrial offgases. The preconditioned fresh CWP scrubbing medium removes sulfur oxide from offgases at such a degree which exceeds the performance of unconditioned, as is CWP two or threefold or even more.